AbstractIn recent years considerable progress has been made in the understanding of the structure and function of the red blood cell membrane. The protein spectrin, of high molecular weight and propensity for self‐association, appears to play a major role, in concert with actin, in maintaining the shape and integrity of the membrane. A study of the physical‐chemical properties of spectrin, and its size, shape, self‐association pattern, and its interaction with other components, leads to a plausible model for the way this protein performs its biological role. The evidence from the structure and interactions of spectrin suggests a structure which is relatively symmetrical yet highly expanded, and which allows extensive, two‐dimensional network formation with actin. In these respects, the structure of spectrin is quite different from that of myosin, to which it has often been likened.